1. Field of the Invention
The present invention relates to a scanning probe microscope.
2. Description of the Related Art
A scanning probe microscope (SPM) is a scanning microscope that mechanically scans a mechanical probe by a scanning mechanism to obtain information of a sample surface. The scanning probe microscope is a generic term for a scanning tunneling microscope (STM), an atomic force microscope (AFM), a scanning magnetic force microscope (MFM), a scanning near field optical microscope (SNOM), and the like. The scanning probe microscope raster scans the mechanical probe and a sample relatively in the X and Y directions to obtain surface information of a desired sample region through the mechanical probe, and map and display it on a monitor TV.
Above all, the AFM is a most popular apparatus, and includes, as main machine mechanisms, a cantilever having a mechanical probe at its free end, an optical displacement sensor to detect the displacement of the cantilever, and a scanning mechanism to relatively scan the mechanical probe and a sample. As the optical displacement sensor, an optical lever type optical displacement sensor is employed most widely because of its simple arrangement and high displacement detection sensitivity. The optical lever type optical displacement sensor applies a beam having a diameter of several μm to several ten μm to a cantilever. A change in the reflection direction of the reflected beam depending on the warp of the lever is detected by a two-segments detector or the like. The operation of the mechanical probe at the free end of the cantilever is detected and output as an electrical signal. While the scanning mechanism is controlled in the Z direction to keep this output constant, the scanning mechanism is similarly scanned in the X and Y directions to map and display the uneven state of a sample surface on the monitor of a computer.
When observing a biological sample in a liquid, the AFM is generally combined with an inverted optical microscope. This is because the inverted optical microscope observation is effective not only when obtaining the finding of a sample, but also when positioning the cantilever at a specific portion of the sample. The AFM often uses a lever scan type scanning mechanism to scan the cantilever in the X, Y, and Z directions so as to cope with various biological samples and sample substrates.
When observing the motion of a biological sample, the observation speed is important in the AFM. For this application, the goal is to obtain one frame within 1 sec, and desirably within 0.1 sec. To increase the speed of the AFM, the machine mechanism is challenging because the electrical circuit of the AFM has already reached a possible level even in an apparatus commercially available at present. Such machine mechanisms are particularly a scanning mechanism having a high scanning speed, a flexible cantilever having a high resonance frequency, and an optical lever type optical displacement sensor capable of detecting the displacement of the cantilever.
For example, when the X direction is the direction of high-speed scanning, the Y direction is the direction of low-speed scanning, and an image of 100 pixels in the X direction and 100 pixels the Y direction is captured in 0.1 sec, the scanning frequencies in the X, Y, and Z directions that are requested of the scanning mechanism reach 1 kHz, 10 Hz, and 100 kHz or more, respectively.
The high-frequency cantilever suited to observe a biological sample requires a spring constant of 1 N/m or less and a resonance frequency of 300 kHz or more. The dimensions of such a cantilever are as small as approximately 1/10 of the dimensions of a cantilever commercially available at present. For example, a cantilever made of silicon nitride has a length of 10 μm, a width of 2 μm, and a thickness of 0.1 μm. The spring constant is 0.1 N/m, the resonance frequency in air is 1.2 MHz, and the resonance frequency in a liquid is approximately 400 kHz.
Further, the optical displacement sensor requires a light converging optical system to change the spot diameter of convergent light to be equal to or smaller than several μm in order to detect the displacement of a very small cantilever.
As described above, it is desirable that high-speed observation of a biological sample by the AFM can be combined with inverted optical microscope observation, that is, the AFM is of the lever scan type. It is necessary that the AFM can use a flexible cantilever having a high resonance frequency, and includes a scanning mechanism to allow high-speed scanning.